1. Field of the Invention
This invention pertains to electrodes for nerves and therapeutic signals to be applied to such electrodes. More particularly, this invention pertains to such electrodes and signals for placement on the vagus nerve for treatment of obesity.
2. Prior Art
a. Neural Conduction Block
The Assignee of the present application has a number of pending U.S. patent applications pertaining to application of a conduction block technology to a nerve for a treatment of a variety of disorders. These applications include the following (all filed Sep. 29, 2003): U.S. patent application Ser. No. 10/674,330, which issued as U.S. Pat. No. 7,489,969 (published Sep. 2, 2004 as Publication No. US 2004/0172086 A1); U.S. patent application Ser. No. 10/675,818 (published Sep. 9, 2004 as US Patent Application Publication No. US 2004/0176812 A1) now abandoned, and U.S. patent application Ser. No. 10/674,324 (published Sep. 2, 2004 as US Patent Application Publication No. 2004/0172085 A1), now abandoned. These patent applications describe, in a preferred embodiment, the application of neural conduction block therapy to a vagus nerve alone or in combination with a stimulation of the nerve.
The conduction block therapy includes application of an electrical signal with parameters selected to down-regulate vagal activity by creating conditions in which normal nerve propagation potentials are blocked at the application of the signal on both afferent and efferent nerves fibers of the vagus. A number of different disorders are identified for treatment through the technique. These disorders include obesity, pancreatitis and other gastrointestinal disorders such as irritable bowel syndrome and functional disorders.
Electrodes may be placed directly on the vagus (for example as cuff electrodes) or may be placed on bands surrounding the vagus at the esophagus or placed on an intraluminal device within the esophagus for transmitting the energy from the device across the tissue of the esophagus to the vagus nerves in the region of the esophagus. These embodiments are disclosed with greater particularity in the Assignee's U.S. patent application Ser. No. 10/752,944 which has issued as U.S. Pat. No. 7,167,750, and Ser. No. 10/752,940, which issued as U.S. Pat. No. 7,444,183, both filed Jan. 6, 2004 with respective publication dates of Aug. 26, 2004 and Sep. 2, 2004, Publication Nos. US 2004/0167583 A1 and 2004/0172088 A1.
b. Blocking Signal Parameters and Duty Cycle
On Jun. 30, 2004 the Assignee of the present application filed Ser. No. 10/881,045 (published Feb. 17, 2005 as Publication No. US 2005/0038484 A1) noting that a duty cycle of electrical impulses to the nerve to block neural conduction on the nerve can be adjusted between periods of blocking and no blocking in order to vary the amount of down regulation of the vagus nerve as well as preventing accommodation by the enteric nervous system.
On Jan. 21, 2005 the Assignee filed Ser. No. 11/040,767 describing with greater particularity parameters for controlling block and to avoid accommodation. That application notes that a representative blocking signal is preferably greater than 500 Hz and that such conduction block is preferably within the parameters disclosed in Solomonow, et al. “control of muscle contractile force through indirect high-frequency stimulation”, American Journal of Physical Medicine, Volume 62, No. 2, pages 71-82 (1983). Particularly, the nerve conduction block is applied with electrical signals selected to block the entire cross-section of the nerve (for example, both afferent, efferent, myelinated and non-myelinated fibers) at the site of applying the blocking signal (as opposed to selected sub-groups of nerve fibers or just afferent and not efferent or vice versa).
Preferably, the frequency of the blocking signal is selected to exceed a 200 Hz threshold frequency described in Solomonow, et al. More preferred parameters are a frequency in excess of 500 Hz (with other parameters as non-limiting examples, being an amplitude of 1-8 mA, pulse width of 100 microseconds, and a duty cycle of 5 minutes on and 5 minutes off. A more preferred blocking signal has a frequency of 3,000 Hz to 5,000 Hz or greater applied by either by bi-polar or mono-polar electrodes. Such a signal has a preferred pulse width of 100 micro-seconds (associated with a frequency of 5,000 Hz).
It is believed this frequency and pulse width best avoid neural recovery from blocking and avoid re-polarization of a nerve. A “short-off” time in the pulse cycle (for example, between cycles or within a cycle) can be acceptable as long as it is short enough to avoid nerve re-polarization. The waveform may be a square, triangular or sinusoidal waveform or other shape. The higher frequencies of 5,000 Hz or more have been found, in porcine studies, to result in more consistent neural conduction block. Kilgore, et al., “Nerve Conduction Block Utilizing High-Frequency Alternating Current”, Medical and Biological Engineering and Computing, Vol. 24, pp. 394-406 (2004). Applicants have determined that a signal amplitude of 0.5 mA to 8 mA is adequate for blocking. However, other amplitudes may suffice.
While a duty cycle can be a predetermined time period, it is currently preferred that the duty cycle be less fixed to reduce the likelihood of patient accommodation whereby the autonomic (parasympathetic, sympathetic and enteric) and/or the central nervous systems accommodates for the loss of signals on the vagus or other nerve. While the periods of off and on can be stable or random, they can be set at any fixed or non-fixed sequence (for example, 5 minutes on followed by 5 minutes off repeated for the duration of the therapy or, alternatively, 5 minutes on followed by 10 minutes off as a first cycle with a following cycle meaning a different set of time—such as 10 minutes on and 2 minutes off, with a non-repeating duty cycle continuing over a 24 hour period). Other signal attributes can be varied to reduce the likelihood of accommodation by the nerve or an organ. These include altering the power, waveform or pulse width.